Unlicensed band communication becomes increasingly important. Examples for unlicensed band communication are wireless networks, e.g., commercial wireless networks, which can operate in unlicensed bands called “Unlicensed National Information Infrastructure” (U-NII) bands.
A popular standard example of wireless networking in unlicensed bands is Wireless LAN-Wi-Fi, IEEE 802.11 (a/b/g/h/j/n). Currently operations bands in Europe are 2.412 GHz to 2.472 GHz, 5.24 GHz to 5.680 GHz. These current wireless LAN (local area network) standards operate in time division duplexing (TDD).
However it is possible in the future that even some licensed bands will be temporarily accessed by secondary operators, making them effectively unlicensed at the physical layer.
In order to enable an efficient link operation, the physical layer (PHY layer) needs to transfer channel state information from the receivers to the sender.
The channel state information of a time division duplexing (TDD) unlicensed band can be estimated via the principle of reciprocity, in which case the channel feedback is conveyed implicitly. However the reliability of this information is arguably unclear.
Otherwise explicit feedback information is by the receiver to the transmitter upon a request from the transmitter, wherein the time slots need to be acquired by the transmitter.
On the other hand, enhanced physical layer concepts such as multi-user MIMO (MIMO=Multiple Input Multiple Output) are already in place for many wireless systems and hence could be utilized for unlicensed band communication. For this purpose, the channel quality or precoding information for multi-user MIMO would have to be made from multiple devices within a short period of time.
Currently for MAC (Media Access Control) operation in unlicensed bands such as in wireless LAN collision avoidance mechanisms by request to send (RTS) and clear to send signals (CTS) are transmitted by the receiver and transmitter respectively. Packet acknowledgments (ACK) are sent in the unlicensed band time-shared with the data payload. In addition, devices can transmit network allocation vector (NA V) in order to reserve the medium which will then be used for transmitting important information such as ACK and HARQ (Hybrid Automatic Repeat Request) packets.
Drawbacks are, for example, that the data transmission of another unexpected wireless link can adversely interfere with the control signaling of the ongoing wireless link in unlicensed band. Moreover, to suppress this interference, channel contention is needed also for control packets. Furthermore, channel quality from each device also needs to done separately in time, thus worsening the problem for multi-user MIMO communication. Moreover, even if the channel quality is conveyed, it has to be received with high reliability at the transmitter. To further complicate things, when the channel is available upon contention, a transmitter can transmit only one or the other (i.e., either control, data or channel quality) at a given time.